There is considerable interest in methods of geotechnical in-situ engineering which enable shear and compression wave velocities in the ground to be accurately estimated, because such measurements provide insight into the response of soil to imposed loads such as buildings, heavy equipment, earthquakes, and explosions. These velocities are desired because they form the core of mathematical theorems which describe the elasticity/plasticity of soils and are used to predict settlement, liquefaction and failure. As such, accuracy in the estimation of shear and compression waves is of paramount importance because these velocities are treated exponentially during the calculation of geotechnical parameters such as the small strain shear modulus, Poisson's Ratio, and Young's Modulus (Ohya, 1982, pp. 1220).
Two methods have developed over the years and are widely used to indirectly estimate these predictive parameters mentioned above: the Standard Penetration Test (SPT) and the Cone Penetration Test (CPT). These tests use empirical correlations, developed with large amounts of data, to relate certain measurements taken during the course of these tests to values for the geotechnical parameters mentioned above. However, these empirical correlation relationships are subject to large variances, and therefore require large factors of safety to be employed when using these values in construction design. Accordingly, industry desired an accurate and reliable method for directly deriving seismic velocities.
For that purpose the Seismic Cone Penetration Test (SCPT) (an extension of the Cone Penetration Test (CPT) was devised to measure seismic velocities directly through data obtained by an installed seismic sensor in the cone penetrometer, in addition to the standard bearing pressure, sleeve friction, and pore pressure sensors. As the cone penetrometer is advanced through the ground, using a pushing force, the advance is halted at one meter (or other such increment) intervals. When the cone is at rest, a seismic event is caused at the surface using a hammer blow or explosive charge, causing seismic waves to propagate from the surface through the soil to be detected by a single seismic sensor installed in the cone penetrometer. This event is recorded and the penetrometer is advanced another increment and the process is repeated. By visually comparing the arrival times of the constituent waveforms integrated in different seismic records obtained from the same probe hole, it is possible to estimate the average velocities of said constituent waveforms over the depth increment under study. This method is known as a pseudo-interval technique in that it utilizes two different seismic events recorded by a single receiver at different depths, versus a true interval technique which would compare the same seismic event utilizing two seismic receivers at different depths.
The determination of velocity from SCPT data is made difficult when low, medium or high frequency noise is present in the recorded traces, and often masks or influences the ideal responses through constructive and/or destructive interference. This problem of noise introduces uncertainty into seismic velocity estimations and results in variances perceived to be as great as those of the empirical correlation methods.